The Syrian Observatory for Human Rights said at least 12 aid workers were killed in the attack

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In this photo provided by the Syrian Civil Defense White Helmets, men stand in rubble after airstrikes hit eastern Aleppo, Syria, Friday, Sept. 9, 2016. The Syrian Civil Defense, also known as the White Helmets, said helicopters dropped crude barrel bombs on the area.

A U.N. humanitarian aid convoy in Syria was hit by airstrikes Monday as the Syrian military declared that a U.S.-Russian brokered cease-fire had failed, and U.N. officials reported many dead and seriously wounded.

The U.S. initially brushed off Damascus' assertions and said it was prepared to extend the agreement, while Russia — after blaming rebels for the violations — suggested it could still be salvaged.

But late Monday, U.S. State Department spokesman John Kirby called the convoy attack an "egregious violation" of the week-long cease-fire and said the U.S. "will reassess the future prospects for cooperation with Russia."

U.N. humanitarian chief Stephen O'Brien said initial reports indicate that many were killed or seriously injured in the convoy attack, including Syrian Arab Red Crescent volunteers.

A Red Crescent warehouse was also hit and a Red Crescent health clinic was reported to be seriously damaged, he said.

O'Brien called the attacks "sickening" and said he was "disgusted and horrified." He stressed that all parties received notification of the convoy, which was carrying aid for about 78,000 people.

U.N. officials said the U.N. and Red Crescent convoy was delivering assistance to the town of Uram al-Kubra, west of Aleppo city. Initial estimates indicated that at least 18 of the 31 trucks in the convoy were hit.

The Britain-based Syrian Observatory for Human Rights said at least 12 were killed in the attack, mostly truck drivers and Red Crescent workers. The Syrian Civil Defense, the volunteer first responder group also known as the White Helmets, confirmed that casualty figure.

They posted images of a number of vehicles on fire in the dead of the night. A video of the attack showed huge balls of fire in a pitch black area, as ambulances arrived on the scene.

Jan Egeland, humanitarian aid coordinator in the office of the U.N. envoy for Syria, told The Associated Press in a text message that the convoy was "bombarded."

Egeland added, "It is outrageous that it was hit while offloading at warehouses."

O'Brien, the U.N. humanitarian chief, said there is no excuse "for waging war on brave and selfless humanitarian workers," and warned that if they were deliberately targeted "it would amount to a war crime."

Elsewhere at least 20 civilians, including a 1-year-old girl, were killed in fresh airstrikes on rebel-held parts of Aleppo city and surrounding areas, according to the Observatory. And Russia said government positions in southwestern Aleppo came under attack from militant groups, including a massive barrage of rockets.

The week-old cease-fire had brought a brief respite to at least some parts the war-torn country.

Its future will be at the top of the agenda of a meeting Tuesday morning of about 20 countries supporting opposing sides in the Syria conflict, including the U.S. and Russia, that belong to the International Syria Support Group.

In the wake of the Syrian military declaration, U.S. Secretary of State John Kerry acknowledged that the first stage of the truce — which called for a week of calm and the delivery of humanitarian aid to several besieged communities — had never really come to fruition. Earlier in the day, Kerry told reporters on the sidelines of the U.N. General Assembly that the truce was "holding but fragile."

The State Department initially said it was ready to work with Russia to strengthen terms of the agreement and expand deliveries of humanitarian aid. Spokesman Kirby called on Russia, which is responsible for ensuring Syria's compliance, to clarify the Syrian position.

A Russian Foreign Ministry statement late Monday night appeared to signal that the deal could still be salvaged, saying that the failure by the rebels in Syria to respect the cease-fire threatens to thwart the agreement.

The cease-fire came into effect on Sept. 12. Under terms of the agreement, the successful completion of seven days of calm and humanitarian aid deliveries would be followed by an ambitious second-stage plan to set up a joint U.S.-Russian coordination center to plan military strikes against the Islamic State group and a powerful al-Qaida-linked militant faction.

But from the start, the truce has been beset by difficulties and mutual accusations of violations.

Aid deliveries to the besieged eastern districts of Aleppo have not reached their destination. The U.N. accused the government of obstructing the delivery while Russian officials said rebels opened fire at the delivery roads.

Rebel forces and activists say government planes have bombed areas that are under the truce agreement, including rebel-held parts of Aleppo.

At least 22 civilians were killed in government bombings over the last week, according to the Syrian Observatory, and four civilians were killed in government-held areas. There were no independent reports of civilian deaths on the government side since the cease-fire came into effect.

By Monday, both the Syrian government and prominent opposition activists were speaking of the truce as if it had already failed.

George Sabra, of the opposition High Negotiations Committee, told The Associated Press on Monday that the truce has been repeatedly violated and did not succeed in its main objective or opening roads for aid.

"Hundreds of thousands of people in Aleppo are waiting for this truce to allow aid to enter the city," he said, adding that aid trucks are still waiting on the Turkey-Syria border. "I believe that the truce is clinically dead."

The Syrian military statement placed the blame on the rebel groups. Damascus refers to all armed opposition groups as terrorists.

"This step (cease-fire) was to constitute a real chance to stop the bloodshed. But the armed terrorist groups didn't take it seriously and didn't commit to any of its articles," the military command statement said. "The armed terrorist groups took advantage of the declared truce system and mobilized terrorists and weapons and regrouped to continue its attacks on civilian and military areas."

A major rebel groups in Syria, Nour el-Din el-Zinki, said soon after the Syrian military declaration that the government, Russia and Iran, another major ally of President Bashar Assad, are responsible for the truce's failure.

"The regime of Bashar Assad had no real intention to commit to the truce. Instead it worked to undermine it with organized violations during the week as well as preventing aid from reaching Aleppo," the group said in a statement sent to reporters.

Earlier Monday, Lt. Gen. Sergei Rudskoi of the Russian military's General Staff said in a briefing that Damascus had fulfilled its obligations.

"With the rebels failing to fulfill conditions the cease-fire agreement, we consider its unilateral observance by the Syrian government forces meaningless," Rudskoi said.

Rudskoi said the rebels violated the truce 302 times since it took effect a week ago, killing 63 civilians and 153 Syrian soldiers. The opposition reported on Monday 254 violations by government forces and their allies since the truce started.

The current tensions come on the heels of the weekend air strike by the U.S.-led coalition on Syrian army positions near Deir el-Zour. Syria and Russia blasted Washington over the attack.

The Saturday airstrikes involved Australian, British and Danish warplanes on Syrian army positions. The U.S. military said it would not intentionally hit Syrian troops, and that it came as it was conducting a raid on IS positions.

Russia's military has said it was told by the Syrian army that at least 62 Syrian soldiers were killed in the Deir el-Zour air raid and more than 100 wounded. The Observatory gave a different death toll, saying 90 troops were killed in the strikes.

Assad said Monday the airstrikes by the U.S.-led coalition against his troops were meant to support the Islamic State group, calling the attack a "blatant American aggression."